Periodontal diseases are diseases in which essentially 100% of the population is "at risk." It has been estimated that 94 million Americans have some form of periodontal disease with 32 million having the advanced disease. In excess of 80% of tooth loss after the age of 35 has been attributed to the chronic, destructive disease process. With the recognition and general acceptance of an etiology based upon the host response to the microbial flora, the primary thrust of research is being directed toward identification of the responsible members of the oral microbiota and characterization of the bacterial cell components promoting the host response. Also of primary importance is the development of objective procedures for early diagnosis of the disease process(es) for the purpose of prevention and treatment. An understanding of the relative importance of the role(s) of the host defense systems and the role(s) of the complex microbial flora as they relate to the mechanism of destructive periodontitis is necessary to progress towards a rational approach to prevention and treatment. Oral Actinomyces spp. have been strongly implicated in the etiology of gingivitis and, possibly, periodontitis. Surface antigenic components will be isolated, purified, and characterized from A. viscosus, A. naeslundii, and A. israelii. The role(s) of these components in the disease process will be evaluated by examining biological correlates of disease; i.e., bone resorption, lymphocyte blastogenesis, macrophage activation, and stimulation of lymphokine synthesis.